Holographic video display system

ABSTRACT

A holographic video display comprises a monochromatic light source, a video signal generator, guided-wave acousto-optic modulators for diffracting light according to signals received from the video signal generator, a vertical scanning subsystem, and an optical path between the acousto-optic modulator and the vertical scanning subsystem. The optical path preferably comprises a Bravais lens system, first and second Fourier transform lens systems, and at least one holographic optical element or stationary mirror of continuous helical shape. In a method for generating a holographic image, monochromatic light is provided to at least one guided-wave acousto-optic modulator, the received light is diffracted according to a video signal, the guided-wave modulator aperture is scanned to produce a holo-line, the motion of the diffraction pattern is undone to render the holo-line stationary, the guided-wave modulator aperture is demagnified to create a wide field of view, and the holo-lines are tiled vertically to create the image.

FIELD OF THE TECHNOLOGY

The present invention relates to holographic devices and, in particular,to a holographic video display based on guided-wave acousto-opticdevices.

BACKGROUND

A fundamental engineering challenge in designing a holographic videodisplay is achieving a high enough space-bandwidth product to meet theimage size and view angle requirements of the viewer. In general, alarge view angle is possible only with very small diffraction fringes(and thus small pixels), while a large image requires a large lightmodulator. In simple terms, what is therefore necessary is a massivenumber of very small pixels. In some cases it is possible to use opticsto trade off one of these for the other, such as, for example, bymagnifying a display that is higher-resolution than needed or bydemagnifying a large modulator to get a small enough effective pixelsize, but passive optics cannot simultaneously increase both size andangle.

Because of the practical limitations on devices that can currently befabricated, it is commonly necessary to use either, or both, scanning(re-using a smaller device for more than one region of the image) ortiling (using multiple copies of a small device) [see, for example, K.Sato, A. Sugita, M. Morimoto, and K. Fujii, “Reconstruction of ColorImages at High Quality by a Holographic Display,” Proc. SPIE PracticalHolography XX, 6136, 2006; C. Slinger, C. Cameron, S. Coomber, R.Miller, D. Payne, A. Smith, M. Smith, M. Stanley, and P. Watson, “RecentDevelopments in Computer-Generated Holography: Toward a PracticalElectroholography System for Interactive 3D Visualization,” Proc. SPIEPractical Holography XVIII, 5290, pp. 27-41, 2004]. For example, Son,Shestak, et al. try to solve the problem of making the scan linestationary while the pattern is traveling across the light modulator byusing a pulsed laser, in order to obtain a “snapshot” of the movingpattern. The resulting scan line is too short, so six snapshots arepasted together end-to-end using six mirrors in a stepped arrangement[J-Y. Son, Jung-Young, S. A. Shestak, S-K. Lee, and H-W. Jeon, “Pulsedlaser holographic video”, Proc. SPIE, vol. 2652, pp. 24-28].

A prior 2-D diffractive display architecture, dating from the 1930s, iscalled the Scophony system. In a 2-D Scophony display, an electricalsinusoidal oscillation is converted to a compression wave that changesthe index of refraction in some material and thus creates a sinusoidalphase grating. Amplitude-modulating this sinusoidal carrier with a videosignal changes the amplitude of a diffracted beam of light, which isthen scanned by rotating or oscillating mirrors to form a video image[H. W. Lee, “The Scophony Television Receiver,” Nature, 142, 3584, pp.59-62, 1938]. Besides the need for a monochromatic light source toenable sharp focus, the major limitation of a Scophony display systemstems from the fact that the grating pattern is moving with the speed ofsound through the diffractive material. To create a stable image, thediffracted light must therefore be imaged in a mirror moving in theopposite direction, a requirement that makes scaling such a systemdifficult.

Two earlier generations of displays developed by the MIT MediaLaboratory were variations on the Scophony system. If a Scophony-typedisplay is not driven with a single amplitude-modulated sinusoid, butrather with a superposition of many gratings at different frequencies,it can output light in multiple directions. The output of anacousto-optic modulator (AOM) can then be treated as one “holo-line” ofa horizontal-parallax-only (HPO) holographic image. The first-generationMIT display, known as the “Mark I” [P. St.-Hilaire, S. A. Benton, M.Lucente, M. L. Jepsen, J. Kollin, and H. Yoshikawa, “Electronic DisplaySystem for Computational Holography,” Proc. SPIE Practical HolographyIV, 1212, pp. 174-182, 1990], is depicted in FIG. 1. As shown in FIG. 1,the Mark I is fundamentally a standard Scophony architecture, with lightfrom laser light source 105 being diffracted by a 50 MHz bandwidth TeO₂AOM 110 driven by a 32,768×192 raster. The video signal employed ismultiplied by a 100 MHz sinusoid and lowpass filtered to retain thelower sideband. The view volume is 25 mm×25 mm×25 mm (W×H×D) and theview angle is 15°. The signal passes through transform lens 120, andthen is scanned by vertical scanner 130 and horizontal scanner 140.Vertical scanner 130 is a galvanometer and horizontal scanner 140 is apolygonal mirror. Holographic image 150 is rendered through output lens160. A Thinking Machines CM2 (not shown) performs the computation.

In order to scale up the image size so that both of a viewer's eyescould fit into the view zone with some added look-around, St.-Hilaire etal. increased the space-bandwidth product of the system by using 18 TeO₂AOM channels in parallel, thus outputting a group of 18 adjacent scanlines, resulting in the “Mark II” architecture [P. St.-Hilaire, S. ABenton, M. Lucente, J. D. Sutter and W. J. Plesniak, “Advances inHolographic Video,” Proc. SPIE Practical Holography VII, 1914, pp.188-196, 1993] shown in FIG. 2. In FIG. 2, the light diffracted fromlaser light source 205 by AOMs 210 passes through transform lens 215 andtoroidal lens pair 220 before being scanned by vertical scanner 230.Vertical scanner 230 moves in 18-line steps to scan out 144 lines, eachhaving 262,144 samples. The view volume is 150 mm×75 mm×150 mm and theview angle is 30°. The signals then pass through vertical relay lenses240, 245 to beamsplitter 250. Because of the difficulty of making asingle horizontal scanner wide enough to meet the requirements, Mark IIuses a synchronized linear array 260 of galvanometric scanners 265.Holographic image 270 is rendered through output lens 280. The 18 videochannels were initially generated by a compact dataflow computer calledCheops [J. A. Watlington, M. Lucente, C. J. Sparrell, V. M. Bove, Jr.,and I. Tamitani, “A Hardware Architecture for Rapid Generation ofElectro-Holographic Fringe Patterns,” Proc. SPIE Practical HolographyIX, 2406, pp. 172-183, 1995], and in later work the display was drivenby three dual-output PC video cards [V. M. Bove, Jr., W. J. Plesniak, T.Quentmeyer, and J. Barabas, “Real-Time Holographic Video Images withCommodity PC Hardware,” Proc. SPIE Stereoscopic Displays andApplications, 5664A, 2005]. The use of parallel AOMs and a segmentedhorizontal scanner gives the Mark II a modular character that allowsscale-up of the system, albeit at the expense of more video inputchannels and more synchronized mirror-drive circuitry.

One goal of research in holographic video has been constructing adisplay suitable for use by consumers. Unlike earlier systems, such adisplay must be at least standard television resolution, quiet,reliable, compact, manufacturable for at most a few hundred dollars, andcapable of being driven by the graphics hardware of a PC or gameconsole, rather than requiring specialized hardware. A vast amount of3-D visual data now exists, particularly in the gaming world (thoughmost is rendered for 2-D viewing), and three-dimensional displays couldeasily take advantage of this resource if they could be manufacturedinexpensively. The widespread adoption of such displays would also sparkinnovation in 3-D capture of real-world scenes.

SUMMARY

In one aspect, the present invention is a holographic video displaysystem that employs a guided-wave device to diffract light from amonochromatic light source. In a further aspect, the present inventionemploys a guided-wave device in conjunction with a stationary helicalmirror or holographic optical element, so that the moving horizontalmirror of the prior art devices is not required.

In a preferred embodiment, the holographic video display systemcomprises at least one monochromatic light source, a video signalgenerator, at least one guided-wave acousto-optic modulator fordiffracting light received from the light source according to at leastone video signal received from the video signal generator, a verticalscanning subsystem, and an optical path for passing the diffracted lightfrom the acousto-optic modulator to the vertical scanning subsystem. Apreferred embodiment of the optical path comprises a Bravais lenssystem, a first Fourier transform lens system, at least one holographicoptical element or stationary mirror of continuous helical shape, and asecond Fourier transform lens system. In one preferred embodiment, thesystem is based on a lithium niobate guided-wave acousto-optic device,which provides twenty or more times the bandwidth of the telluriumdioxide bulk-wave acousto-optic modulators used in previous displays.The system architecture and the guided-wave device are preferably drivenby a graphics chip.

In another aspect, the present invention is a method for displayingholographic images. In a preferred embodiment, monochromatic light isprovided to at least one guided-wave acousto-optic modulator, thereceived light is diffracted according to at least one video signal, theguided-wave modulator aperture is scanned to produce a holo-line, themotion of the diffraction pattern is undone to render the holo-linestationary, the guided-wave modulator aperture is demagnified to createa wide field of view, and the holo-lines are tiled vertically to createthe holographic image.

The present invention has been implemented as a new holo-video displayarchitecture known as the “Mark III”. It reduces the cost and size of aholo-video display, making it into an inexpensive peripheral to astandard desktop PC or game machine that can be driven by standardgraphics chips. The novel display architecture of the present inventioneliminates the high-speed horizontal scanning mechanism that hastraditionally limited the scalability of Scophony-style video displays.

BRIEF DESCRIPTION OF THE DRAWINGS

Other aspects, advantages and novel features of the invention willbecome more apparent from the following detailed description of theinvention when considered in conjunction with the accompanying drawings,wherein:

FIG. 1 is a schematic depicting the prior art Mark I architecture;

FIG. 2 is a schematic depicting the prior art Mark II architecture;

FIG. 3 is a schematic depicting a preferred embodiment of thearchitecture of the holographic video display system of the presentinvention; and

FIG. 4 is a schematic depicting how, in one aspect of a preferredembodiment of the present invention, a set of video channels controlshorizontal diffraction of light passing through the guided-waveacousto-optic device and another provides vertical diffraction.

DETAILED DESCRIPTION

The present invention is a new holo-video display architecture, thepreferred embodiment of which is known as the “Mark III”. The presentinvention employs a guided-wave device to diffract light from amonochromatic light source. The architecture also eliminates thehigh-speed horizontal scanning mechanism that has traditionally limitedthe scalability of Scophony-style video displays, performing thefunction continuously with a helical mirror or a holographic opticalelement (HOE) and thereby replacing the moving horizontal mirror of theprior art devices. The system architecture and the guided-wave deviceare preferably driven by a graphics chip. In a preferred embodiment, thesystem employs lithium niobate guided-wave acousto-optic devices, whichprovide twenty or more times the bandwidth of the tellurium dioxidebulk-wave acousto-optic modulators of the previous displays. The basicapproach used ithe present invention is also appropriate for the designof compact and inexpensive 2-D video projection applications.

Because the prior art Mark II has a modular architecture, that systemmay be scaled to allow very large view volumes. However, the Mark IIsystem is already expensive and physically large (about the size of adining table top), so a departure was made from a direct extrapolationof Mark II's design and instead the display of the present invention iscentered on a single, inexpensive, very high bandwidth light modulatorand a novel optical design that eliminates the horizontal mirror and asmany other optical elements as possible. The result is a complete,packaged display system that is capable of being driven by one(dual-output) PC video card.

A light-modulation technology that is particularly suitable for use inthe present invention is the guided-wave acousto-optic modulator (AOM),frequently also referred to as a guided-wave scanner (GWS) [C. S. Tsai(ed.), Guided-Wave Acousto-Optics, Springer-Verlag, Berlin, 1990]. Aguided-wave scanner is easily made from a slab of lithium niobate(LiNbO₃) that has been acid-treated to create a subsurface waveguidethrough proton exchange and then patterned on the surface with aluminumtransducers. This simple device may be produced in quantity at pricesapproaching those of the rather similar surface acoustic wave (SAW)devices currently on the market for a few dollars, can have over 1 GHzof usable bandwidth, can diffract light along two axes [V. V. Proklovand E. M. Korablev, “Multichannel Waveguide Devices Using CollinearAcousto-optic Interaction,” Proc. IEEE 1992 Ultrasonics Symposium, pp.173-178, 1992. This paper also references an earlier (1981) paper inRussian reporting on the authors' work in this area. See also C. S.Tsai, Q. Li, and C. L. Chang, “Guided-Wave Two-Dimensional Acousto-OpticScanner Using Proton-Exchanged Lithium Niobate Waveguide,” Fiber andIntegrated Optics, 17, pp. 157-166, 1998], and can rotate thepolarization of the diffracted light so that the undiffracted portioncan be blocked with a polarizer. Although the vertical diffraction angleavailable is perhaps too small to be usable for the vertical scanning ofa video display, it is applied in conjunction with holographic opticalelements in the present invention to solve the horizontal scanningproblem inherent in past Scophony-architecture displays.

The fundamental requirement for the optical design of the presentinvention is that the diffracted light from the modulator be placed atthe correct position over time to present a proper display. The opticsmust scan the guided-wave scanner aperture to produce a holo-line, undothe motion of the diffraction pattern to render the holo-linestationary, demagnify the guided-wave scanner aperture to create a widefield of view, and tile the holo-lines vertically to create a rasterimage. In a preferred embodiment, the optics comprise a Bravais lenssystem, a modified telephoto Fourier transform system, two holographicoptical elements (HOEs), a demagnifying transform lens, and a verticalscanning subsystem.

FIG. 3 is a schematic depicting a simple embodiment of the architectureof the holographic video display system of the present invention. InFIG. 3, light from monochromatic laser 305 is diffracted by guided-waveAOM 310 through modified Bravais lens system 313. In the embodimentshown, Bravais lens system 313 comprises two lenses 315, 317, but otherconfigurations known in the art would be suitable. The diffracted lightthen passed through a first Fourier transform lens system, depicted inthis embodiment as telephoto horizontal transform lens 320, then throughfirst holographic element (HOE) 330, a second Fourier transform lenssystem, which in this embodiment is a demagnifying horizontal transformlens 340, and second holographic element 345. The signal then passes tothe vertical scanning subsystem comprising, in the embodiment of FIG. 3,vertical scan transform lens 350, vertical scanner 360, and verticalscan output lens 370. Holographic image 380 is then rendered throughdiffuser 390.

Diffractive displays require illumination by a monochromatic lightsource such as a laser, or multiple such sources if a full-color imageis desired. In the preferred embodiment, the light modulator isilluminated by a semiconductor laser. It is generally necessary to blockthe undiffracted light at the exit of the light modulator and pass onlythe diffracted light to the optical path. This can be done by placing astop at the output of the modulator, or, because the diffracted andundiffracted light have different polarizations, a polarizer can beplaced on the light source and another on the output of the modulator,suitably oriented so as to pass only the diffracted light.

In a preferred embodiment, the guided-wave scanner is composed of twosets of aluminum interdigital transducers that straddle a region ofproton-exchanged lithium niobate. The scanner uses two sets oftransducers to create surface acoustic waves that first deflect lighthorizontally, via Bragg diffraction, and then vertically by means ofmode-conversion. The device achieves Bragg diffraction through a set ofphased-array transducers that launch a holographic pattern of acousticwaves at the Bragg angle of the light traveling in the waveguide.Because these transducers each have several phase-shifted acousticemitters, they are able to steer the acoustic pattern to meet the Braggangle of light over an angular range corresponding to an acousticbandwidth of 200 MHz per transducer. A second set of simple (not-phased)transducers creates a pattern of sound waves that meets light travelingin the waveguide “head-on.” Over a particular range of acousticfrequencies, this collinear interaction can “bump” the light into aleaky mode via polarization-rotating mode conversion. This leaky-modelight passes through the waveguide interface and finally exits from theedge of the substrate. This second, collinear interaction can be used toscan light vertically over an angle corresponding to approximately 70MHz of acoustic bandwidth. It will be clear to one of skill in the artof the invention that, while particular materials and structures aredescribed for use in this embodiment, any of the many equivalentmaterials and structures known in the art will also be suitable and maybe advantageously employed in the present invention. For example, thereare many other materials besides lithium niobate that are known in theart to be suitable for guided-wave devices.

FIG. 4 is a schematic depicting how, in a preferred embodiment of theguided-wave scanner of the present invention, multiple video channelscontrol horizontal diffraction of light 405 passing through waveguide410 and another channel provides vertical diffraction. As shown in FIG.4, each of five horizontal transducers has a bandwidth of 200 MHz and acenter frequency of (200n+160) MHz for 1≦n≦5. In each case, one of thevideo channels 420 taken from graphics processor 430, which has a 400MHz pixel clock and thus the signals have a 200 MHz bandwidth, isupconverted 440 to a lower-single-sideband by any of the many means wellknown in the art. Vertical transducer 460 has a bandwidth of 70 MHz anda center frequency of 460 MHz, so lower-single-sideband carrierfrequency 470 is set to 495 MHz. Rendering for this display is verysimilar to the method discussed in V. M. Bove, Jr., W. J. Plesniak, T.Quentmeyer, and J. Barabas, “Real-Time Holographic Video Images withCommodity PC Hardware,” Proc. SPIE Stereoscopic Displays andApplications, 5664A, 2005, though here each channel does not represent aseparate scan line, but rather all channels must carry differentfrequency ranges (and thus different diffraction angles) for the samescan line. It will be clear to one of skill in the art of the inventionthat many other frequencies and many other ways of generating therequired single-sideband video signals are known in the art and aresuitable for use in the present invention.

A preferred embodiment of the guided-wave scanner is constructed byproton exchanging a region of the LiNbO₃ substrate to create a surfacewaveguide and then patterning transducers. The proton exchange step isusually accomplished by masking the substrate with SiO₂, and thenimmersing the substrate in a >200° C. melt of benzoic acid for a timeperiod ranging from a few minutes to a few hours, depending on thedesired waveguide depth. In this case, the substrate is immersed in a250° C. melt for 30 minutes for a waveguide depth of approximately 1micron. Finally, the SiO₂ mask is removed and aluminum transducers arephotolithographically placed on the proton exchanged LiNbO₃ substrateusing a negative resist lift-off process. More details regarding thedevice fabrication process may be found in D. E. Smalley, “IntegratedOptics for Holographic Video,” M. Eng. Thesis, Massachusetts Instituteof Technology, Cambridge Mass., 2006. Several iterations of guided-wavescanner design have been fabricated and tested, and device testingindicates that these devices meet the target requirements for thedisplay system.

As previously discussed, the optics preferably comprise a Bravais lenssystem, a modified telephoto Fourier transform system, two holographicoptical elements (HOEs), a demagnifying transform lens, and a verticalscanning subsystem. The Bravais system magnifies the guided-wavescanner's vertical scan angle while forcing the scan to still appear tocome from the guided-wave scanner location. This provides verticalmagnification without the need to relay the horizontal image, therebymaintaining the overall optical path length. This also means that thehorizontal and vertical images of the object remain collocated and thehorizontal and vertical optical paths do not need to be handledseparately, permitting the use of spherical lenses instead of separatecylindrical lenses for each axis. The typical Bravais system will causea collimated input beam to diverge, so the Bravais system is modified sothat the beam remains collimated. A field lens is placed at/near thescanning plane to counteract the beam divergence, reducing the verticalcollimated beam height and thereby recollimating the output beam, whilenot affecting the scanner's vertical scan angle. In operation, the lightemitted from the guided-wave scanner first passes through the modifiedBravais system in order to magnify the guided-wave scanner's verticalscan angle by 10×, forcing the scan to appear to come from theguided-wave scanner's position. As a consequence of this effect (andunlike in Mark II), the horizontal and vertical scans both appear toemanate from the same location The light then passes to the modifiedtelephoto Fourier transform lens system.

In the preferred embodiment, the modified telephoto Fourier transformsystem converts the linear motion of the traveling diffraction fringepattern into rotational motion that can be descanned by the horizontalscanner, thereby allowing the fringes to be descanned later by anoptical element that creates a reverse rotation. The telephoto system ismodified from a typical system in order to reduce the overall length ofthe optical path for efficient physical packaging of the system. Atelephoto arrangement with multiple elements is used, since a singlelens does not have a sufficient focal length. A typical telephotoarrangement of a separated diverging and converging pair of lenses, whencompared to a single lens of the same effective focal length, has adecreased front focal length but a greatly increased back focal length.The overall optical path length of a telephoto arrangement from objectto image is much larger than the overall path length of thecorresponding single lens. A field lens is therefore added to thetypical telephoto arrangement to reduce the back focal length, therebymaking the optical system compact. The addition of the field lensat/near the object reduces the back focal length of the telephotoarrangement, but leaves its effective focal length the same. Thetelephoto arrangement also reduces the front focal distance and allowsfine-tuning of the focal length by adjusting the spacing between thetelephoto elements.

In the preferred embodiment, the optics also include HOEs that work inconjunction with the guided-wave scanner's vertical scan capability toscan the guided-wave scanner aperture, making stationary and trackingthe holographic fringes without moving parts. Since the holo-line ishorizontal parallax only, no image information is carried vertically,and the vertical direction can be temporarily used to encode the desiredguided-wave scanner's aperture position along the holo-line. The firstHOE simultaneously scans the guided-wave scanner aperture, which isnarrower than a full holo-line. The first HOE is designed so that theamount of horizontal deflection varies continuously with verticalposition (analogous to a mirror with a helical surface, but transmissiverather than reflective). The guided-wave scanner's aperture is scannedvertically onto the HOE, and the HOE then scans the aperturehorizontally. The vertical scan rate, and therefore the horizontal scanspeed, is adjusted to track the motion of the holographic fringesrendering them stationary. The HOE is followed by a transform lens inorder to convert the rotational scan of the guided-wave scanner apertureinto a linear motion and form a holo-line. The transform lens alsomagnifies the holo-line's field of view. A second HOE then s thevertical encoding (vertical scan component) introduced earlier by theguided-wave scanner's vertical scanner. This solid-state scanningfeature results in a more robust, inexpensive, and scalable system thandesigns using the traditional Scophony solution of moving mirrors.

The prototype system is designed for operation with 510-532 nmsemiconductor laser illumination. In the prototype, a complete, packagedmonochrome display system is capable of being driven by one(dual-output) PC video card. The target specifications for the firstsystem were 440 scan lines, 30 Hz, 24° view angle, 80 mm×60 mm×80 mm(W×H×D) view volume, and approximately 1.5 m total optical path length,folded to fit into a relatively shallow box. Further generations of thisdesign increase the view volume and view angle, and add full color.

In the prototype, a single NVIDIA Quadro FX 4500 graphics processorperforms the rendering and fringe computations and generates the videosignals for the Mark III display. Mark III treats six video lines of4096 samples as a single holo-line of 24,576 samples, and thus mustdivide the horizontal sync signal by six before using it to advance theposition of the vertical scanner. Because the prototype display ismonochrome, the dual RGB outputs of the graphics chip are treated as sixindependent frame buffers operating with 400 MHz pixel clock (and thus200 MHz of bandwidth). Five of these channels drive the horizontaltransducers of the GWS with the image information for each holo-line,and the sixth drives the vertical transducer with a fixed pattern oneach holo-line consisting of a sinusoid whose frequency linearlyincreases from the beginning of the holo-line to the end. The startingfrequency and chirp rate of this sinusoid can be changed in software toadjust the “horizontal hold” implemented by the HOE discussed in thepreceding section.

While a specific preferred embodiment is disclosed herein, it will beclear to one of ordinary skill in the art that many variations on thearchitecture of the preferred embodiment described herein are suitableand may be advantageously employed in the present invention. Forexample, the light source needs only to be monochromatic rather thancoherent, so it does not need to be a laser; it could be, for example,but not limited to, a gas-discharge tube, an LED, or a filteredincandescent or arc source. The illumination laser may be fabricatedon-chip. Light may be coupled, for example, via a grating coupler, end-or butt-coupled, or evanescently, such as with prisms. The system canprovide full color by employing red, green, and blue light sourcessimultaneously or by employing a light source having a color thatchanges over time. The device may integrate nonlinear elements, such as,but not limited to, a sum/difference amplifier or an Optical ParametricAmplifier/Generator for on-chip Red, Green and Blue creation from someother wavelength or wavelengths.

There may further be several light modulator devices in a stack tohandle multiple light sources, or a single faster one may be used tocycle through multiple light sources over time. The laser or other lightsource may be integrated with the modulator in a single unit. The lightmodulator may have several guided-wave devices in a single substrate tomake a wider screen (deflecting to a wider horizontal angle by havingeach device do an angular subsection of the screen). The devices may bestacked not just vertically, but also horizontally, and may befabricated on the same chip. The chip may employ multiple light sourcesand the horizontal transducers need not be of different centerfrequency. Some, or all, of the geometric optics may be fabricated onthe lithium niobate chip, including the horizontal and vertical scannersand polarizers. The vertical deflection may be created by a separateguided-wave scanning device from the device that creates the horizontaldeflection, thus requiring two one-dimensional devices rather than asingle two-dimensional device. The guided wave scanner may also be usedfor 2D or 1D projection. The waveguide may also be created by metalindiffusion, such as Ti or Zn indiffusion, by ion implantation, or bythe deposition of, or growth of, another layer of material such assilicon dioxide or zinc oxide. It may also be created with laser orthermal modification of the surface via the photorefractive effect. Thewaveguide may also be created by metal indiffusion, such as Ti or Znindiffusion, by ion implantation, or by the deposition of, or growth of,another layer of material such as silicon dioxide or zinc oxide. It mayalso be created with laser or thermal modification of the surface viathe photorefractive effect.

Further, there are a broad range of ways to generate the video signalsother than by using a graphics chip. The RF modulation may not be donein analog after the video signal generation, but rather the signals maybe generated directly at appropriate RF frequencies. There might not bemultiple video inputs to the horizontal scanner at different centerfrequencies, but rather a single one with a wider frequency range.

There are also alternate lens arrangements that would work—the preferredembodiment being an attempt to optimize for total path length. There aretherefore many other ways known in the art to build the optical path,including through the use of convex and concave mirrors. For example,the first holographic optical element may be a helical mirror instead.The second holographic optical element may also not be necessary,replacing it instead with a simple vertical diffuser. The HOE may be avolume hologram. The input light in that case may be angularlymultiplexed instead of vertically multiplexed.

The horizontal scanner may be a helical mirror (reflective), a twistedprism (transmissive), or a set of orthogonally crossed concave andconvex lenses (or mirrors). The horizontal scanners may be eithercontinuous or discrete. The horizontal scanner may be a diffractive orholographic optical element (DOE or HOE). This HOE would be a grating ofvertically varying pitch. The resulting optical element would appear tobe a family of hyperbolas with their asymptotes on the major axes. Thephase of the HOE's grating can also be vertically varying (perhapsrandomly varying), thereby reducing the effect of the inherentcross-coupled scanning behavior of the horizontal scanner. The HOE maybe produced optically (interfering diverging and converging beams) orcomputationally. A multiplicity of scanners may be used in a horizontalarrangement to reduce the effects of the inherent cross-coupled scanningbehavior of the horizontal scanner. The horizontal scanner may be ahorizontal parallax only (HPO) hologram of the above-mentioned scanners,thereby removing the inherent cross-coupled scanning behavior. Thetraditional optical arrangement for producing hpo holograms is modifiedusing a vertical (elliptical) diffuser either in front of (preferred) orbehind the physical horizontal scanner. The HOE's grating may becomputationally Fourier-filtered to remove the inherent cross-coupledscanning behavior. These apply to the second scanner (located at thefirst AOM image plane to descan the AOM's vertical multiplexing) aswell. The second scanner may be a masked elliptical diffuser.

The inherent cross-coupling behavior in the horizontal scanner may beremoved by relaying the horizontal component of the beam (for example,using a 4f cylindrical lens system), while reflecting (flipping orinverting) the beam's vertical component at the 4 f system's transformplane onto a second helical mirror. The relaying of the horizontalcomponent of the beam causes the output horizontal scan angle to bedoubled, while the retroreflecting of the beam's vertical componentcauses the output vertical deflection to be removed. Reflection of thebeam's vertical component can be achieved with a negative index ofrefraction metamaterial, a volume hologram, or be approximated by astack of spaced horizontal mirrors. A vertical stack of vertical onlyretroreflectors can be placed at the transform plane of the 4 f system,thereby removing the need for the second lens of the 4 f system and thesecond helical mirror. These vertical only retroreflectors may bemicrocorner reflectors (two long mirrors oriented at right angles) ormicrorods embedded in a matrix where the index of refraction of themicrorods are twice that of the matrix or a single rod surrounded by acylindrical mirror such that the focal point of the cylindrical mirroris coincident with the axis of the rod or a vertical stack of such rods.

Horizontal scanning may alternatively be performed using cross-firedsurface acoustic waves (SAW) instead of using an HOE. The saturation ofthe output light may be controlled via piezo elements on an input fiberor by an on-chip electro-optic modulator or by any of the acousto-opticmodulators. The chip may be elongated and the SAW transducers replicatedto create pixels and then used without needing horizontal de-rotation byeither pulsing the laser or by using a continuous laser that is gated byelectro-optic modulators behind every pixel, so that the light dutycycle is multiplied by N, where N is the number of pixels. The basisfringes may be hard wired into the horizontal transducers. The SAWdevice may utilize other types of surface or near-surface wavesincluding SH-SAW, Love Waves or leaky SAW waves. The X-cut, Y-cut orZ-cut of lithium niobate may be used. The exchange technique may be PE,DMPE, APE, SPE, RPE, HTPE, TIPE, DE, TIDE, or any combination.

In alternate configurations, the vertical scanner may be electronic,such as, but not limited to a micro-electromechanical (MEMS) device,rather than being a galvanometer. The vertical scanner may be a motorsynchronized with the video timing. The vertical scanner may use a prismor mirrored polygon instead of a flat mirror. The vertical scan may beused to mode-convert light into guided modes, leaky modes, or intofreespace modes. Freespace modes would not have the same bandwidthlimitations as leaky modes. The vertical scanner may be collocated/nearthe horizontal scanner, thereby removing the separate vertical scanningsection and reducing overall length. The vertical scanner may be apaddle type, so that although the vertical scanner is displaced from thehorizontal scanner, the vertical scanner's effective center of scan iscollocated with the horizontal scanner. Continuous or discrete mirrorsor prisms may be used in conjunction with the diffuser to reduce thediffusion angle required, providing morelight throughput. The displaymay be used to write a pattern onto an OASLM that serves as the finaloutput surface. The display may be used to write the back of aphotosensitive film or photosensitive surface, thereby creating ahardcopy hologram printer (or a 2D or 1D printer).

While a preferred embodiment is disclosed, many other implementationswill occur to one of ordinary skill in the art and are all within thescope of the invention. Each of the various embodiments described abovemay be combined with other described embodiments in order to providemultiple features. Furthermore, while the foregoing describes a numberof separate embodiments of the apparatus and method of the presentinvention, what has been described herein is merely illustrative of theapplication of the principles of the present invention. Otherarrangements, methods, modifications, and substitutions by one ofordinary skill in the art are therefore also considered to be within thescope of the present invention, which is not to be limited except by theclaims that follow.

What is claimed is:
 1. A holographic video display system comprising: atleast one monochromatic light source; a video signal generator; at leastone guided-wave acousto-optic modulator for diffracting light receivedfrom the light source according to at least one video signal receivedfrom the video signal generator; a vertical scanning subsystem forrendering a holographic image; and an optical path for passing thediffracted light from the acousto-optic modulator to the verticalscanning subsystem, the optical path comprising: a Bravais lens system;a first Fourier transform lens system; at least one holographic opticalelement or at least one stationary mirror of continuous helical shape;and a second Fourier transform lens system.
 2. The holographic videodisplay system of claim 1, wherein the guided-wave acousto-opticmodulator is a two-dimensional device.
 3. The holographic video displaysystem of claim 1, wherein the vertical scanning subsystem furthercomprises a galvanometric scanner.
 4. The holographic video displaysystem of claim 1, wherein the vertical scanning subsystem furthercomprises a motor.
 5. The holographic video display system of claim 1,wherein the vertical scanning subsystem further comprises amicro-electromechanical scanner.
 6. The holographic video display systemof claim 1, further comprising a polarizer that blocks undiffractedlight from exiting the guided-wave mosulator.
 7. The holographic videodisplay system of claim 1, wherein the first Fourier transform lenssystem is a telephoto lens system.
 8. The holographic video displaysystem of claim 1, wherein the second Fourier transform lens systemperforms demagnification.
 9. The holographic video display system ofclaim 1, wherein the guided-wave acousto-optic modulator furthercomprises video signal inputs for multiple frequency bands.
 10. Theholographic video display system of claim 1, wherein the color of themonochromatic light source changes with time in order to create atime-sequential full-color display.
 11. The holographic video displaysystem of claim 1, wherein there are three monochromatic light sources,one light source each being red, blue, and green, and three guided-wavemodulators, each guided-wave modulator receiving light from one of thethree monochromatic light sources.
 12. The holographic video displaysystem of claim 1, wherein there are three monochromatic light sources,one light source each being red, blue, and green, and the guided-wavemodulator sequentially cycles through receiving light from each of thethree monochromatic light sources.
 13. A holographic video imageproduced using the display of claim
 1. 14. A method for generating aholographic image, comprising: providing monochromatic light to at leastone guided-wave acousto-optic modulator; diffracting the received lightaccording to at least one video signal; scanning the aperture of theguided-wave acousto-optic modulator to produce a holo-line; undoing themotion of the diffraction pattern to render the holo-line stationary;demagnifying the guided-wave acousto-optic modulator aperture to createa wide field of view; and tiling the holo-lines vertically to create theholographic image.
 15. The method of claim 14, wherein the guided-waveacousto-optic modulator is a two-dimensional device.
 16. The method ofclaim 14, wherein the step of tiling the holo-lines vertically isperformed via a galvanometric scanner, motor, or micro-electromechanicalscanner.
 17. The method of claim 14, further comprising the step ofusing a polarizer to block undiffracted light exiting the guided-wavemodulator.
 18. The method of claim 14, wherein the step of demagnifyingis performed via a Fourier transform lens system.
 19. The method ofclaim 14, wherein the guided-wave acousto-optic modulator receives videosignal inputs for multiple frequency bands.
 20. The method of claim 14,further comprising the step of changing the color of the monochromaticlight source with time in order to create a time-sequential full-colordisplay.
 21. The method of claim 14, wherein there are threemonochromatic light sources, one light source each being red, blue, andgreen, and three guided-wave modulators, each guided-wave modulatorreceiving light from one of the three monochromatic light sources. 22.The method of claim 14, wherein there are three monochromatic lightsources, one light source each being red, blue, and green, and furthercomprising the step of sequentially cycling the guided-wave modulatorthrough receiving light from each of the three monochromatic lightsources.
 23. A holographic video display that employs the method ofclaim 14.